Alzheimer's Disease (AD) is the most common cause of dementia, the fourth leading cause of death, and has enormous economic and emotional costs for caregivers of patients. Most of the patients with AD develop disruptive, agitated behaviors at some point during the ailment that are a common catalyst for placement in long-term care settings where they represent more than half of all the residents. Disruptive behaviors create stress for both staff and patients in long-term care settings, reduce patient quality of life because of excess use of physical restraints, and drain the financial resources of the facilities. This study proposes to design specific interventions designed to decrease disruptive, agitated behavior in patients with AD residing in long-term care facilities. This 12-week study will assess the efficacy of two pharmacological agents, an atypical neuroleptic (risperidone) and an anticonvulsant and mood stabilizer (gabapentin) while identifying moderators of differential treatment response to the two drugs. Specifically, this study aims to randomize 130 patients with AD who are manifesting agitated disruptive behavior, into two groups of 65 patients each, with one group receiving risperidone and the other gabapentin. There are two main hypotheses: 1) Patients in both of the treatment groups will manifest different overall group decreases in ratings of disruptive, agitated behavior; and 2) Measurable, clinical characteristics will identify subgroups of subjects with differential treatment response to the two medications. The two variables to be considered as moderators are as follows: a) Patients with higher psychosis ratings will manifest greater responsivity to risperidone compared to gabapentin, and b) Patients with high levels of affective lability will manifest greater responsivity to gabapentin compared to risperidone.